Proton transfer from carbon. A study of the acid–base-catalysed relaxation and the bromination of aryl-substituted methanedisulfones

Abstract

Seven aryl-substituted derivatives of bis(ethylsulfonyl)methane have been synthesized and studied thermodynamically and kinetically in aqueous solution with respect to proton abstraction from the central carbon atom. The acidities of the six carbon acids are in the range pK_A 10–12. The relaxation times of the reactions were studied by a spectrophotometric temperature-jump technique (hydroxide ion and phenolates as proton acceptors) and are in the microsecond range. The rates of bromination of the various carbanions have also been studied (potentiometrically) and the reactions are shown to be almost diffusion-controlled, with no marked trend with aromatic substituent. A small chemical activation may be due to a steric effect by the two sulfonyl groups. The rates of deprotonation, however, show considerable chemical activation, i.e., intrinsic rates are 3–4 orders of magnitude lower than for normal acids, indicating partial delocalization of the negative charge on the carbanion. This effect is somewhat larger than for cyano-activated carbon acids but considerably smaller than for carbon acids activated by carbonyl or nitro groups. It is concluded that sulfonyl groups interact with carbanions primarily by polar effects, but also, to some extent, mesomerically. The degree of mesomerism is, however, uncertain because part of the intrinsic barrier may derive from steric effects, as is also observed for bromination.